Mary Warren undergoes a significant change in demeanor in act 2 of The Crucible, after she has participated in the court proceedings of the witch trials. Though we don't see much of Mary in act 1, the little that is shown suggests that, prior to the trials, she is a timid, anxious girl, easily controlled by others. Despite initially urging Abigail to tell the truth about what they were doing in the woods to avoid a harsher sentence, she quickly bends to Abigail's will and agrees to play along with her lies. When John Proctor admonishes her for being out with the girls, she does not even reply but wordlessly obeys his orders to return home. She is shown as meek and submissive.

However, by act 2, things have changed. Elizabeth tells John that Mary went to Salem despite John having forbidden her. When Elizabeth tried to stop her, Mary would not back down.

ELIZABETH: I forbid her go, and she raises up her chin like the daughter of a prince, and says to me, "I must go to Salem, Goody Proctor; I am an official of the court!"

This is not the same Mary Warren we saw in act 1. Despite being the Proctors' serving girl, she now sees herself as having equal stature in the community due to her involvement in the trials. When she returns from the trials, she becomes proud and outspoken when John questions her, defending her actions and the purpose of the trials.

Much of this comes from the fact that Mary is either playing her part in Abigail's elaborate lie incredibly well, or she has actually started to believe that they are rightly accusing witches. When she arrives home from the trial, she claims to be weak from the efforts of the day and genuinely seems quite ill to begin with. She fervently tells the Proctors about Sarah Good's wickedness and remains undeterred when Elizabeth says that Mary never mentioned anything about Sarah Good trying to hurt her before.

MARY: I never knew it before. I never knew anything before. When she come into the court I say to myself, I must not accuse this woman, for she sleep in ditches, and so very old and poor. But then—then she sit there, denying and denying, and I feel a misty coldness climbin‘ up my back, and the skin on my skull begin to creep, and I feel a clamp around my neck and I cannot breathe air; and then—entranced—I hear a voice, a screamin‘ voice, and it were my voice—and all at once I remembered everything she done to me!

The stage directions that Miller has added here seem to suggest that Mary truly believes what she is saying. She thinks that she and the other girls are helping root out an evil in their community and protecting their neighbors. She is incredibly righteous in her efforts. When John continues to doubt her and threatens to whip her, she does not cower as she had before.

MARY: I'll not stand whipping any more! ... The Devil‘s loose in Salem, Mister Proctor; we must discover where he's hiding!

She goes on to announce that she defended Elizabeth, showing that she is only interested in uncovering those who are actually guilty.

However, this entire show of assertion and devotion to the trials is put to the test when Elizabeth is accused after being found with the doll that Mary gave her. When John insists that Mary go to court and tell the truth about the doll, Mary refuses. She is still able to stand up to John, but the real cause of her newfound confidence is revealed. She is only strong with Abigail's support, and revealing the truth about Elizabeth would anger Abigail. When faced with this prospect, Mary is still the timid, submissive girl we met in act 1. However, as long as she is in Abigail's good graces, she knows she has the social power to defy Proctor.

This balking to tell the truth at Elizabeth's trial also suggests that even if Mary has come to believe the lies she has told herself, some part of her knows it is all an illusion that could easily come crashing down, implicating her in the process. She is terrified to reveal that the real evil in Salem is not the devil but rather Abigail's wrath.

Prior to act 2, Mary Warren is depicted as a timid, frightened teenager who fears Abigail Williams's retribution. Similar to the other girls, Mary Warren is too passive and weak to challenge Abigail and agrees to corroborate her fabricated story involving witchcraft. Despite her timid nature and meek personality, Mary Warren appears bold and resolute in act 2. When John Proctor arrives home, Elizabeth explains how Mary Warren defied her orders to remain home and proudly announced that she was an official of the court.

After John argues with Elizabeth about exposing Abigail as a fraud, Mary Warren arrives home and seems upset about the proceedings. When John challenges the validity of the court and instructs Mary to stay home, she once again displays her recently acquired self-esteem by telling him,

I must tell you, sir, I will be gone every day now. I am amazed you do not see what weighty work we do.

Mary defiantly challenges John and insists on attending the witch trials because she is an official of the court. Prior to her being named an official, Mary Warren was a quiet, submissive teenager who would never think of standing up to John. The entire community now looks to her for guidance, and she feels entitled to disobey him.

When John threatens physical violence, Mary brings up the fact she saved Elizabeth in court but refuses to name Abigail as her accuser. After John instructs Mary to go to bed, she responds by saying,

I‘ll not be ordered to bed no more, Mister Proctor! I am eighteen and a woman, however single!

Her elevated status in the community has given her a false sense of confidence, and she attempts to wield her authority in John's home. Despite her newly acquired assertive personality, Mary Warren cowers when John makes her testify against the girls, and she eventually succumbs to Abigail's pressure.

Initially, Mary Warren comes across as a very shy, timid creature, someone easily intimidated and controlled by others. In act 2 of The Crucible, John Proctor seeks to take advantage of this by getting Mary to agree to tell the truth in open court: that all the hysteria is based on nothing but lies and that all that dancing in the forest that she did with the other girls had nothing to do with witchcraft.

For good measure, he also wants her to testify that she was the one who made the doll, supposedly a means of transmitting an evil curse, and stuck a pin in it. This is to save Elizabeth's life, as she's now been accused of using the doll to do harm to Abigail Williams by stabbing it with a pin.

But Mary won't play ball. Truth be told, she's scared stiff of what Abby will do to her if she spills the beans. Ironically, then, she's being more assertive towards John while at the same time remaining in a state of frightened passivity in relation to Abigail.

Mary may be scared of Abigail, but she still feels powerful by associating with the instigator of the witch trials, who has herself become a powerful figure in town. This is a whole new experience for Mary, and she's not about to end it just yet, no matter how hard John pleads with her to stand up and tell the truth.

One of the most evident changes in Mary is that she is more assertive.  In Act I, when Proctor enters and threatens to beat her, she is much more timid.  In Act II, she presents herself as much stronger and assertive because of her role in the trials.  Simply put, Mary has power and she is not afraid to show it.  The fact that she tells Proctor that she is tired and that she cannot be bothered with all that she has to do is reflective of this.  Proctor himself is surprised by her newly found sense of power.  Another change in Mary is that she shows herself to be more aware of the dynamic between Proctor and Abigail.  This comes out in the end of the Act and in doing so, she reveals herself to be more direct and Proctor begins to become more solidified in his belief of bringing down Abigail.  The ending of the act forces an eventual confrontation where Mary has to choose between Proctor and Abigail.  This is something that she did not have to choose in the first Act, and is something that ends the act with her wails and screams.  It is here where another change is evident in that it is recognizable that she is the weak link between Abigail and her circle of "mean girls."  I think that this is another change in Mary in the second act.